Glossary · Insulation & windows
R-value
Also called: R-values · thermal resistance · R per inch · R-value per inch · effective R-value
How well a material resists heat flow; higher is better, and insulation is rated with it, such as R-20 for walls or R-50 for attics.
What it means
R-value measures resistance to conducted heat. Thicker insulation has a higher R-value, and materials differ in R per inch (see blown-in insulation, spray foam and rigid foam). Real performance is lower when insulation is compressed, has gaps or is bypassed by air leakage or thermal bridging.
Why it matters in an audit
Auditors compare your insulation with recommended or code levels to decide where more will pay off; attics are often the easiest place to add a lot.
Typical numbers
Reference values with their sources. Your auditor’s report gives the figure for your own house.
| What | Value | Source |
|---|---|---|
| ENERGY STAR retrofit target, uninsulated attic | Zone 1: R-30; zones 2–3: R-49; zones 4–8: R-60 (US IECC zones) | energystar.gov ↗ |
| ENERGY STAR target, attic with 3–4 in. already | Zone 1: R-25; zones 2–3: R-38; zones 4–8: R-49 | energystar.gov ↗ |
| ENERGY STAR target, floors | R-13 (zones 1–2); R-19 (3, 4A, 4B); R-30 (4C, 5, 6); R-38 (7–8) | energystar.gov ↗ |
| 2021 IECC ceilings, new homes | R-49 in zones 2–3; R-60 in zones 4–8 | energycodes.gov ↗ |
| Estimating R-value from depth (ENERGY STAR) | Per inch: loose fiberglass ≈ 2.5; rock wool ≈ 2.8; cellulose ≈ 3.7; fiberglass batts ≈ 3.2; vermiculite ≈ 2.7 | energystar.gov ↗ |
In the United States
ENERGY STAR's recommended levels are keyed to IECC climate zones and based on the 2021 IECC.